Electrostatic speakers are attracting attention as speakers capable of generating sound with high straight forwardness by emitting plane waves.
An electrostatic speaker has a structure in which, on each side of a sheet-like oscillator having conductivity, a sheet-like electrode having conductivity is disposed with an insulating spacer held therebetween. The electrode is required to be equipped with numerous through-holes passing through the inside and outside faces thereof and serving as air moving paths so as not to obstruct the oscillation of the oscillator, and the electrode is made of, for example, a cloth woven from conductive fibers or a punching metal sheet.
When a direct-current bias voltage is applied between the oscillator and each of the two electrodes, and an alternate-current voltage drive signal corresponding to a sound waveform is applied between the two electrodes, a drive force corresponding to the drive signal is generated between the oscillator and the electrode in accordance with Coulomb's law, and the oscillator oscillating between the two electrodes by virtue of the drive force generates sound pressure. As a result, sound corresponding to the sound waveform is emitted.
In order that the electrostatic speaker having the above-mentioned structure reproduces sound with a large volume, a high voltage is required to be applied between the oscillator and the electrode; however, if the applied voltage is too high, discharge, such as arc discharge, occurs between the oscillator and the electrode, or the oscillator and the electrode make contact with each other via the through-holes in the spacer disposed therebetween, whereby leakage may occur in some cases.
If discharge and leakage occur between the oscillator and the electrode, explosive sound is superimposed on reproduced sound as noise, and the oscillator and electrode are broken, whereby deterioration in the sound reproduction performance of the electrostatic speaker occurs.
For the purpose of solving the above-mentioned problems, it is considered that, for example, the surface of the electrode is subjected to an insulation treatment; however, this method generally increases cost.
Hence, for example, in Patent Document 1, an electrostatic speaker has been proposed in which a capacitor element is disposed in series between the electrode and the supply source of the drive signal, thereby suppressing the occurrence of discharge or the like at lower cost than that of the treatment for insulating the surface of the electrode.
Furthermore, for example, in Patent Document 2, an electrostatic speaker has been proposed in which a sheet-like water repellent member having water repellency is disposed on the outside of each of the two electrodes thereof, and a surface member provided with numerous small-diameter through-holes is disposed further outside each of the water repellent members, whereby liquid and solid are hard to enter the inside.
With the electrostatic speaker proposed in Patent Document 2, moisture and dust are hard to enter the inside, whereby the induction of discharge and leakage due to moisture and dust is reduced.